Nurr1 is a transcription factor required for the acquisition and maintenance of the dopaminergic phenotype of the neurons of the substantia nigra. In nurr1 knockout mice, neurons of the substantia nigra do not develop a dopamine phenotype. Conversely, overexpression of nurr1 in embryonic stem cells is sufficient to produce a dopaminergic phenotype. This may be due to the ability of nurr1 to regulate expression of the tyrosine hydroxylase and dopamine transporter genes. Polymorphisms or mutations of nurr1 have also been implicated to play a role in familial Parkinson's disease. Nurr1 expression has also been shown to decrease throughout the course of normal human aging and after cocaine abuse. In addition to full-length nurr1, two alternative isoforms (nurr1b and TINUR) have been identified by in human tissues, and two different isoforms (nurr1a and nurr2) have been identified in mouse tissues. These isoforms are derived from alternatively spliced mRNAs with internal deletions, or truncated forms lacking the sequence encoding the amino- or carboxy- terminus. As both termini contain activation function domains, loss of either or both could dramatically affect nurrl activity. While multiple splice variants of nurr1 have been previously reported, the described variants have been cloned from different tissues, most of which were non-neuronal and all of which were non-dopaminergic. In Aim 1 of this proposal, the nurr1 isoform expression profile will be defined in rat ventral mesencephalon (substantia nigra and ventral tegmentum) and all identified forms will be cloned. In Aim 2, the relative activity of the nurrl variants will be delineated in a dopaminergic cell line. In Aim 3, rats will be treated with cocaine to determine if cocaine alters total nurr1 expression and if expression of the individual isoforms is differentially regulated. These studies will lead to a greater understanding of nurr1's role in the maintenance of the dopamine phenotype, which may ultimately enable the development of new therapeutic agents to treat the loss of dopamine phenotype associated with Parkinson's disease and cocaine abuse.